1. Field of the Invention
The field of the invention relates to non-noble metal catalysts for the oxygen reduction reaction including methods of manufacture.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) in which an electrolyte in the form of an ion-exchange membrane is disposed between two electrode layers. The electrode layers are made from porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. In a typical MEA, the electrode layers provide structural support to the membrane, which is typically thin and flexible.
The MEA contains an electrocatalyst, typically comprising finely comminuted platinum particles disposed in a layer at each membrane/electrode layer interface, to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
A significant emphasis has been placed to reduce MEA costs by reducing the platinum loading while maintaining or even improving performance and reliability. One approach is to eliminate platinum entirely and replace it with a cheaper alternative catalytic material. In particular, a significant amount of work has been done working on replacing platinum for the oxygen reduction reaction at the cathode.
Aside from cost, platinum catalysts have a further disadvantage when used in direct methanol fuel cells (DMFCs) in which methanol is used as the fuel. Namely, platinum at the cathode oxidizes methanol that crosses over from the anode leading to depolarisation and hence serious power losses in the cell.
Bron et al. (Journal of Electroanalytical Chemistry 500, 2001, 510–517) discloses a ruthenium-based catalyst for oxygen reduction. The catalysts were prepared by reacting Ru3(CO)12 with selenium for 20 hours in deaerated xylene under reluxing conditions. The product was filtered, washed with diethylether and dried in an oven at 90° C. to produce a black powder. Bron et al. studied the effect of selenium and found a maximum benefit at about 15 mol % Se though catalytic activity was still observed in a selenium free catalyst. Bron concluded that the catalytic center in the selenium-containing catalyst is different from the catalytic center in the selenium-free catalyst. Selenium was also found to protect the catalyst against electrochemical oxidation and therefore led to enhanced stability.
In a second publication produced by the same group, Tributsch et al. (Journal of Applied Electrochemistry 31, 2001, 739–748), found that heating of this product resulted in the loss of carbon species in well defined steps. The first step involved the loss of CO and CO2 between 250 and 350° C. and a second step was observed at temperatures above 600° C. Further, Tributsch et al. found a loss of catalytic activity resulting from the release of carbon species at elevated temperatures. This led Tributsch et al. to propose a complicated catalytic structure comprising a cubane-like organometallic ruthenium-complex on the surface of a ruthenium nanoparticle doped with a chalcogen (selenium or sulfur). Inspiration for this model appears to be an iron hydrogenase from the Clostridium pasteurianum bacterium.
In a prior study on a related system, namely a MoRuS and MoRuSe system, Trapp et al. (J. Chem. Soc, Faraday Trans. 92(21), 1996, 4311–4319) arrived at significantly different conclusions. In the synthesis carried out by Trapp et al., Ru3(CO)12 and Mo(CO)6 were refluxed in xylene with sulfur or selenium for 20 hours. The catalyst powder was then filtered and dried at room temperature before being introduced into a tubular furnace at 350° C. for one hour. Though Trapp et al. also performed a heating step, instead of reduced catalytic activity as reported by Tributsch et al. supra, Trapp et al. observed improved activity. In fact, such heating step was referred to as “catalyst activation.” In addition, Trapp et al. concluded that the Ru species is the active center of the catalyst with some synergistic effects observed between the ruthenium and the molybdenum sites in the mass-transport region. Trapp et al. also found that catalytic activity of the MoRuS was not affected by methanol. Under conditions of simulated methanol cross-over, the activated MoRuS catalyst had a similar activity to platinum. However, similar activity was only observed with methanol present. In the absence of methanol, the activity of activated MoRuS catalyst was significantly worse than platinum.
Despite considerable efforts, a non-noble metal-based catalyst with activity similar to platinum has yet to be developed. In addition, existing synthetic methodologies are directed to experimental scale and, as such, are not necessarily amenable to commercial scale production. For example, metal carbonyls, which are typically used as starting materials, are relatively expensive and typical solvent systems used, namely xylene, are toxic and environmentally damaging. Thus, even if the catalysts were suitable for use in fuel cells, an environmentally friendly synthetic method would be needed.
The present invention fulfills these and other needs and provides further related advantages.